The next generation mobile wireless communication system (5G) or new radio (NR), may support a diverse set of use cases and a diverse set of deployment scenarios. The later includes deployment at both low frequencies (100 s of MHz), similar to existing (Long Term Evolution) LTE systems, and very high frequencies (e.g., mm waves in the tens of GHz). Similar to LTE, NR may use Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (i.e., from a network node, gNB, eNB, or base station (BS), to a wireless device (WD)). In the uplink (i.e., from wireless device to network node), both OFDM and Discrete Fourier Transform (DFT)-spread OFDM (DFT-S-OFDM), also known as single-carrier frequency division multiple access (SC-FDMA) in LTE, may be supported.
The basic NR physical resource can be seen as a time-frequency grid similar to the grid in LTE as illustrated in FIG. 1, which is a block diagram of LTE physical resources, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. Although a subcarrier spacing of Δf 15 kHz is shown in FIG. 1, different subcarrier spacing values are supported in NR. The supported subcarrier spacing values (also referred to as different numerologies) in NR are given by Δf (15×2α) kHz where a is a non-negative integer.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RBs), where a resource block corresponds to one slot (0.5 ms) in the time domain and twelve contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. For NR, resource allocation is described in terms of resource blocks in frequency domain and OFDM symbols in time domain. A resource block in NR may also be twelve subcarriers in frequency. A RB is also referred to as physical RB (PRB) herein. In the time domain, downlink and uplink transmissions in NR may be organized into equally-sized subframes similar to LTE as shown in FIG. 2, which is a block diagram of the LTE time-domain structure with 15 kHz subcarrier spacing. In NR, a subframe may be further divided into multiple slots of equal duration. Data scheduling in NR can be either on a subframe basis as in LTE, or on a slot basis. In NR, subframe length may be fixed at 1 ms regardless of the numerology used. In NR, the slot duration for a numerology of (15×2α)kHz may be given by ½α ms assuming 14 OFDM symbols per slot, and the number of slots per subframe depends on the numerology. For convenience, subframe is used herein.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the gNB transmits downlink control information (DCI) about which data is to be transmitted to and which resource blocks in the current downlink subframe the data is transmitted on. This control signaling is typically transmitted in the first one or two OFDM symbols in each subframe in NR. The control information may be carried on Physical Downlink Control Channel (PDCCH) and data may be carried on Physical Downlink Shared Channel (PDSCH). A wireless device first detects and decodes PDCCH and if a PDCCH is decoded successfully, the wireless device decodes the corresponding PDSCH based on the decoded control information in the PDCCH.
Uplink data transmissions may also be dynamically scheduled using PDCCH. Similar to downlink, a wireless device first decodes uplink grants in PDCCH and then transmits data over the Physical Uplink Shared Channel (PUSCH) based the decoded control information in the uplink grant such as modulation order, coding rate, uplink resource allocation, and etc.
Spatial Multiplexing
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance may be in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO. A core component in LTE and NR is the support of MIMO antenna deployments and MIMO related techniques. Spatial multiplexing is one of the MIMO techniques used to achieve high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 3, which illustrates an exemplary transmission structure of precoded spatial multiplexing mode in LTE.
As seen in FIG. 3, the information carrying symbol vector s≤[s1, s2, . . . , sr]T is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. Each symbol in s [s1, s2, . . . , sr]T corresponds to a MIMO layer and r may be referred to as the transmission rank. In this way, spatial multiplexing may be achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (RE). The number of symbols r is typically adapted to suit the current channel properties.
The received signal at a UE with NR receive antennas at a certain RE n is given byynHnWs+en where yn is a NR×1 received signal vector, Hn a NR×NT channel matrix at the RE, en is a NR×1 noise and interference vector received at the RE by the UE. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective, i.e., different over frequency. The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the wireless device. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the wireless device, the inter-layer interference may be reduced.
The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. The transmission rank may also be dependent on the Signal to Noise plus Interference Ratio (SINR) observed at the wireless device. Typically, a higher SINR is required for transmissions with higher ranks. For efficient performance, it may be important that a transmission rank that matches the channel properties as well as the interference is selected.
Channel State Information Reference Signals (CSI-RS)
In LTE, CSI-RS was introduced for channel estimations in the downlink for transmission modes 9 and 10. A unique CSI-RS may be allocated to each network node transmit antenna (or antenna port) and may be used by a UE to measure downlink channel associated with each of transmit antenna ports. CSI reference signals are defined for one to 32 antenna ports. The antenna ports sometimes are also referred to as CSI-RS ports. CSI-RS are transmitted in certain REs and subframes. FIG. 4 is a block diagram of REs available for CSI-RS allocation in each PRB in LTE. FIG. 4 shows the REs available for CSI-RS allocations in each PRB in LTE. Up to 40 REs can be configured for CSI-RS.
For two antenna ports, a CSI-RS for each antenna port may be allocated with two REs in the same subcarrrier and in two adjacent OFDM symbols in each PRB. CSI-RS signals for two antenna ports are multiplexed using a length two orthogonal cover codes (OCC), also referred to as OCC2. Thus, for 2 antenna ports, there are 20 different patterns available within a subframe. FIGS. 5 and 6 show an example of CSI-RS resource for 2 and 4 ports in LTE, respectively.
By measuring CSI-RS, a wireless device can estimate the channel the CSI-RS is traversing including the radio propagation channel and antenna gains. This type of CSI-RS may also be referred to as Non-Zero Power (NZP) CSI-RS. In addition to NZP CSI-RS, Zero Power (ZP) CSI-RS was introduced in LTE. ZP CSI-RS may be defined on one or more 4-port CSI-RS resource. The purpose was to indicate to a wireless device that the associated REs are muted at the network node. If the ZP CSI-RS may be allocated to be fully overlapping with NZP CSI-RS in an adjacent cell to improve channel estimation by wireless devices in the adjacent cell since there is no interference created by this cell. An example of ZP-CSI-RS resource is shown in FIG. 7, where the ZP CSI-RS occupies 8 REs per PRB (i.e., two 4-port CSI-RS resources). FIG. 7 is therefore a block diagram of NZP CSI-RS and ZP CSI-RS.
In LTE release (Rel) 11, CSI interference measurement (CSI-IM) resource was also introduced for a wireless device to measure interference. A CSI-IM resource may be defined as a 4-port CSI-RS resource that may be also fully overlapped with ZP CSI-RS. A CSI process may be defined by a NZP CSI-RS resource for channel estimation and an CSI-IM resource for interference and noise estimation. A wireless device can estimate the effective channel and noise plus interference for a CSI-RS process and consequently determine the rank, precoding matrix, and the channel quality. FIG. 8 is a block diagram of NZP CSI-RS, ZP CSI-RS and CSI-IM.
CSI Feedback
For CSI feedback, LTE has adopted an implicit CSI mechanism where a wireless device feedback of the downlink channel state information is in terms of a transmission rank indicator (RI), a precoder matrix indicator (PMI), and one or two channel quality indicator(s) (CQI). The CQI/RI/PMI report can be wideband or frequency selective depending on which reporting mode that is configured. The RI corresponds to a recommended number of layers that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoder. The CQI represents a recommended modulation level (i.e., QPSK, 16QAM, etc.) and coding rate for each transport block. LTE supports transmission of one or two transport blocks (i.e., separately encoded blocks of information) to a wireless device in a subframe. There is thus a relation between a CQI and an SINR of the spatial layers over which the transport block or blocks are transmitted.
Beamformed CSI-RS
The beamformed (or precoded) CSI-RS concept was introduced in LTE, in which a CSI-RS is precoded and transmitted over more than one antenna port. This is in contrast with non-precoded CSI-RS in which each CSI-RS is transmitted on one antenna port. Beamformed CSI-RS can be used when the direction of a wireless device or wireless devices is roughly known so that CSI-RS can be transmitted in a narrow beam or beams to reach the wireless device or wireless devices. This can improve CSI-RS coverage with increased beamforming gain and also reduce CSI-RS resource and CSI feedback overhead.
The non-precoded CSI-RS based feedback is referred as “CLASS A” CSI feedback, while beamformed CSI-RS operation is referred to as “Class B” CSI feedback. In CLASS B CSI feedback, a wireless device can be configured with up to 8 CSI-RS resources (i.e., multiple CSI-RS beams), each with up to 8 ports. The wireless device reports back a CSI-RS resource indicator (CRI) to indicate the best beam and the corresponding CQI, RI, PMI within the selected beam.
A wireless device configured for Class B operation with one CSI-RS resource of up to 8 ports is a special case, in which each CSI-RS port may correspond to a particular beam. In that case, a wireless device may be configured to use a port selection and combining codebook.
Hybrid Class A and Class B CSI reporting may also be supported. In one scenario, Class A is used to identify the approximate direction of a wireless device while Class B is used to “fine tune” the CSI.
MU-MIMO
When all the data layers are transmitted to one wireless device, it may be referred to as single user multiple-in multiple-output or SU-MIMO. On the other hand, when the data layers are transmitted to multiple wireless devices, it may be referred to as multi-user MIMO or MU-MIMO. MU-MIMO is possible when, for example, two wireless devices are located in different areas of a cell such that they can be separated through different precoders (or beams) at the network node, e.g., eNB/gNB. The two wireless devices may be served on the same time-frequency resources (e.g., PRBs) by using different precoders or beams. MU-MIMO may require much more accurate downlink channel information than in SU-MIMO in order for the network node to use precoding to separate the wireless devices, i.e., reducing cross interference to the co-scheduled wireless devices. For that purpose, advanced CSI feedback was introduced in LTE in which a new codebook was defined trying to capture more accurate downlink channel information. In NR, a type II codebook may be designed for the same purpose.
MU-MIMO Interference
In MU-MIMO, in addition to interference from other cells, also referred to as inter-cell interference, interference among UEs participating in MU-MIMO may also be experienced by the wireless devices, also referred to as intra-cell interference or MU interference. MU interference may be more difficult to measure or estimate due to the dynamic nature of transmissions to wireless devices paired in MU-MIMO. Assuming there are K+1 wireless devices sharing the same time-frequency resources in a data transmission, the received signal at the kth (k1, 2, . . . , K+1) wireless device and at the ith RE can be expressed asyk(i)=Hk(i)Wk(i)sk(i)+Hk(i)Σm≠kK+1Wm(i)sm(i)+ek(i)
where Hk(i), Wk(i), sk(i) are the channel matrix, the precoding matrix and the data vector associated with the kth wireless device at the ith RE. MU interference experienced at the kth wireless device may be expressed asIMUk=Hk(i)Σm≠kK+1Wm(i)sm(i)
and ek(i) may be the noise plus inter-cell interference received at the kth wireless device. Only ek(i) is typically considered in the existing LTE CSI feedback. Existing CSI reporting defined in LTE is mainly for SU-MIMO operation, in which a wireless device is configured with one CSI-RS resource for channel measurement and one CSI-IM resource for interference measurement. With the number of supported antenna ports increasing in both LTE and NR, supporting MU-MIMO becomes even more important. Existing CSI feedback in LTE may not be sufficient to support MU-MIMO. Theoretically, ZP-CSI-RS can be used by eNB/gNB to emulate MU-MIMO interference by injecting MU interference in the ZP CSI-RS resource. However, a separate ZP CSI-RS may be generally needed for each wireless device and when many wireless devices are participating MU-MIMO, the required ZP CSI-RS resources can be large. Unfortunately, this may significantly increase the overhead of ZP CSI-RS.